Although conventional ceramic production techniques date to the Sumerians, rudimentary efforts to harden natural raw materials nearly 5000 years ago, the 20th Century witnessed the emergence of a new kind of ceramics. Today, advances in machinery permit the creation of “engineering ceramics” which are advanced products formed by using high temperatures to harden inorganic nonmetallic compounds such as oxides, nitrides, borides, carbides, silicides, and sulfides into materials with extraordinary properties. Some engineered ceramics exhibit extreme heat resistance, with melting temperatures as high as 4000° F. and sulfides into materials with extraordinary properties. Additionally, engineered ceramics are exceptionally hard; cubic boron, for example, is almost as hard as diamond. Finally, engineered ceramics are lightweight, intrinsically strong, and resistant to corrosive chemicals. In short, engineered ceramics are in high demand in the energy, aviation, and automobile industries, where high gas prices and environmental concerns are motivating engineers to create engines and power plants that can operate at very high temperatures while consuming less fuel.
An advanced process for forming ceramics known as “gelcasting”, developed by the Oak Ridge National Laboratory (ORNL) during the 1980's was an important first step in the creation of high-quality, complex-shaped engineering ceramic parts, and has become a widely used process in ceramics engineering with advantages over traditional methods such as slip casting, pressure casting, die pressing, extrusion, and injection molding. Put simply, traditional casting processes are expensive and produce parts with a high rate of defects. Injection molding, for example, begins with the mixing of a ceramic powder with a polymer or other binder to form a very thick liquid that is subsequently forced, or injected, into a mold under very high temperatures and pressures (and at great expense). Other processes, such as slip casting, did not produce parts that shrink uniformly in the firing process, thereby producing inferior parts. Additionally, in all traditional casting processes, defects in sintered parts necessitate expensive machining—using diamond tools to cut, shape, and finish the parts—before the product is capable of use.
Gelcasting, in contrast, mixes a powder with a gel precursor mix (typically including a multifunctional acrylate, such as the monomers acrylamide or methacrylamide) to create a ceramic slurry that can be poured—instead of injected under high pressure and temperature—into a mold. The use of monomers, instead of polymers, lets engineers load high amounts of solids into a slurry that, once dried in the mold, produces a gel that is strong enough to resist crumbling while soft enough to be manipulated in its “green body” phase without the use of expensive diamond cutting tools in the machining process. Once the gel is fired, or sintered, a gelcast product can take on intricate mold characteristics in producing very strong and heat resistant parts.
Unfortunately, even gelcast engineered ceramics can be brittle and continue to face obstacles in obtaining a low cost manufacturing method. In particular, the high temperatures and pressures needed to sinter—firing a powder or other mass of fine particles at high temperatures to create a hardened product—ceramics are presently cost-prohibitive, unsafe, and ineffective in providing reliably strong parts. Moreover, specific inorganic materials, such as silicon carbide, are prone to corrosive buildups and have granular shapes that inhibit the sintering process. The sintering of silicon carbide is presently only capable of being affected by hot press sintering; however, even hot press sintered silicon carbide has yet to reach a sufficiently satisfactory theoretical density. There is a need in the art of the sintering process that utilizes affordable machinery, streamlined processes, and produces part shapes that are not hot pressed.
The present disclosure solves these problems, as further detailed below, by commencing operations with a ceramic powder free of the corrosive buildups that normally inhibit the sintering process. Sintering operations are further improved by the application of vibrations—at critical process stages—to produce a compacted work product of superior hardness and strength. Finally, the improved processes described herein can be carried out with less expensive, more reliable, and dramatically safer induction heating crucibles.